Wireless devices often do not have equal range in all directions. This is typically due to the anisotropy of the antennas used by the wireless devices. Anisotropy means that a feature is directionally dependent or non-uniform in all directions. In other words, an antenna is able to transmit or receive data at longer distances in some directions and at shorter distances in other directions.
Poor performance of a radio in indoor and industrial environments can be caused by various factors, some of which are corrected using different techniques. For example, shadow fading is caused by obstacles between a transmitter and a receiver. Spacing two or more antennas far apart from each other and using a diversity technique can help reduce shadow fading. Fading because of destructive interference can be reduced using two or more antennas deployed at a quarter wavelength distance along with a diversity technique. A null in an antenna limits the antenna's range in the direction of the null. This can be handled by deploying another antenna that has better gain in the direction of the null and using a diversity technique. Signal attenuation at an antenna can be caused by mismatched polarization, which can be reduced using multiple antennas at opposite polarizations (such as orthogonal polarizations with a linear antenna) along with a diversity technique. Fading caused by multipath interference can be reduced using a directional antenna or an antenna array with beam steering.
As can be seen here, deploying multiple antennas can help reduce many problems associated with wireless communications. However, some of these techniques have opposing requirements (such as far separation versus quarter wavelength separation). As a result, there are multiple ways to utilize two or more antennas to provide antenna diversity.
ZigBee devices are wireless devices that operate according to the ZigBee standard, which is a low-power wireless networking standard. In addition to the above problems with antenna performance, the ZigBee standard places an artificial limitation on devices by limiting when one device can communicate with another device. More specifically, a “child” node typically joins an existing ZigBee network through a “parent” node, and the ZigBee standard defines a “link cost” associated with a link between the child and parent nodes. The link cost is associated with the probability that a message from the child node will be successfully received by the parent node, where a lower cost indicates a higher likelihood of success. The ZigBee standard requires that the link cost be less than or equal to a value of three before allowing a child node to join or rejoin a network through a particular parent node. As a result, poor wireless link qualities caused by any of the various problems noted above can limit link costs, reducing or even preventing ZigBee devices from joining or rejoining a network.